Electrophotography is a technique for obtaining images by developing an electrostatic latent image formed on a photosensitive member with a toner for developing electrostatic latent image comprising a colored particle and, if necessary, one or more kinds of other particles such as an external additive and/or a carrier, transferring the toner onto a recording medium such as paper or an OHP sheet, and fixing the transferred toner onto the recording medium so as to obtain a printed product.
In the case of full-color electrophotography, colors are generally reproduced using three color toners of primary colors, yellow, magenta, and cyan or four color toners of these three colors and black to form color images. For example, in the case of color copy, an original color document is scanned, broken into a plurality of pixels, and converted into digital image signals for different colors, and then a charged photosensitive member is irradiated with light corresponding to the digital image signal for each color to form an electrostatic latent image. Then, the electrostatic latent image for a first color is developed with a toner of the first color on the photosensitive member, and a resulting toner image is transferred onto a recording medium such as paper or an OHP film. These developing and transfer steps are repeated for a second color, a third color, and, if necessary, a fourth color to superimpose color images on top of each other on the same recording medium while these color images are adjusted to their respective correct positions. In the case of color printing, digital signals for four colors are directly transmitted from, for example, a computer to a photosensitive member to separately form electrostatic latent images for four different colors. Then, development is carried out for each color and a resulting toner image is transferred onto a recording medium in the same manner as in the case of color copy. After the completion of development with all the four color toners and transfer, the toner images of four colors are fixed onto the recording medium by one-time fixing to form a full-color image.
Known methods for fixing toner images include heating, pressing, heating and pressing, and exposure to solvent vapor. Among them, a heating and pressing method using a heat roller is most widely used.
Here, in the field of such electrophotography, particularly in color electrophotography using two or more color toners, there is a strong demand for higher printing speed and lower energy for fixing. Therefore, a toner is required to have a low minimum fixing temperature and a high hot offset generation temperature. In addition, sharpness of an image formed at a higher printing speed with lower fixing energy is of growing importance.
It is generally known that a reduction in the glass transition temperature or melt viscosity of a toner is effective at lowering its minimum fixing temperature.
However, such a method may cause the following various problems: (1) the shelf stability of a toner is impaired; (2) a toner remaining on a photosensitive member forms a thin film, that is, filming occurs; (3) the maximum number of sheets that can be printed is reduced due to the occurrence of fog or filming and the printing durability of a toner is reduced; and (4) a hot offset phenomenon (hereinafter, simply referred to as “hot offset”) occurs at low temperature so that part of a toner forming a toner image transferred onto a recording medium is attached to a heat roller and is then again transferred onto a subsequent recording medium.
Such hot offset is conventionally prevented by applying silicone oil onto a fixing roller.
However, such a method makes it difficult to reduce the size and price of a fixing unit (e.g. an image-forming apparatus). In addition, there is also a problem that an obtained image is too glossy.
In recent years, from the viewpoint of improving the low-temperature fixability of a toner and preventing the occurrence of hot offset, toner has been studied by focusing attention on its viscoelasticity.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2004-151638 discloses a color toner for developing electrostatic image, which is designed to have excellent low-temperature fixability, high resistance to hot offset and excellent shelf stability. This color toner for developing electrostatic image comprises at least a binder resin, a colorant, and a charge control agent, and has the following dynamic viscoelastic characteristics as measured at a frequency of 10 Hz and a strain of 1%: a storage modulus at 80° C. (G′80) from 5×106 to 5×108 Pa; a storage modulus at 160° C. (G′160) from 1×102 to 1×104 Pa; a maximum value of loss tangent (tan δ) from 1.2 to 2.5 in the range from 80 to 100° C.; and a minimum value of tan δ of 1 to 2 in the range from 95 to 125° C., and further has a volume average particle diameter (Dv) from 2 to 11 μm, and a ratio between volume average particle diameter (Dv) and number average particle diameter (Dp) (i.e. Dv/Dp) of 1.3 or less.
Further, JP-A No. Hei. 5-100477 discloses a toner designed to have excellent low-temperature fixability and offers resistance to offset over a wide temperature range. The toner comprises at least a binder resin having, as dynamic viscoelastic behaviors, a dynamic loss G″ at 150° C. of 1×105 dyn/cm2 or less and a dynamic modulus G′ at 200° C. of 2×104 dyn/cm2 or more and a release agent having a melt viscosity at 140° C. from 5 to 100 cps, and has a loss tangent (tan δ) in the range from 150 to 200° C. from 0.05 to 1.0.